Lava Milling Stations can be found here and there on the planet, but it isn't like you can reach one a few days after leaving another. It can take months to find one, and that's if you know how to get there. It is very easy to get lost on a planet like Mustafar, so if you go off in the wrong direction, you will probably die before you even get close to one, and your chances are bad enough if you know where your'e going. Milling Stations provide bases for operation during war, but even then, they still provide fire-based materia and mako-stones, as well as energy crystals for lightsabers. The stations recieve the nessecary imports for the survival of every species in the galaxy, so staying at one station is a preferred choice compared to roaming around.